The present invention relates to an ion concentration meter, and specifically, relates to an ion concentration meter which can measure a minute change in ion concentration at an extremely high accuracy and which is suitable for detection of a change in ion concentration, a leakage of ions or the like in various apparatuses or various systems.
Measurement of the concentrations of ammonium, sodium, chloride, calcium, potassium, carbonate, silica, magnesium, sulphate ions and the like may be required in various industrial fields. For example, in a cooling water producing system, as described later, heat exchange is carried out between the side of a refrigerator and brine used for respective use points by a heat exchanger, and the cooled brine is stored in respective target tanks and used as cooling water at the respective use points. In such a system, particularly, because leakage of ammonia from the refrigerator side into the brine through the heat exchanger, etc. poses a problem, it is required to measure and monitor the concentration of the ammonia which has leaked into the brine. It is known that the concentration of ammonia in a sample has a correlation with the conductivity of the sample, and that it is effective to measure the conductivity of the sample for determining the concentration of ammonia.
Generally, in a conventional method for measuring a concentration of ammonia in a sample, for example, an aliquot amount of sample is collected for measurement, ammonia in the sample is evaporated by heating or using a strong alkali and the evaporated ammonia is trapped in deionized water, and the concentration of ammonia and the change of the concentration are detected by measuring a change in conductivity of water. In this method, sampling, cleaning, water for trapping, etc. are necessary, and there is a possibility that an apparatus for this method may become extremely expensive for achieving a high-accuracy measurement though it depends upon the performance of a conductivity meter.
Further, in a case where a conventional-type conductivity meter is used and when a base conductivity of a sample is very great, it is impossible to detect a minute change in concentration of ammonia. For example, assuming that the conductivity of a sample having a base conductivity of 3000 xcexcS is changed by an amount of 0.5 xcexcS by adding ammonia, the ratio of change in conductivity is about {fraction (1/1000)}, and it is impossible to measure such a change by a conventional-type conductivity meter in view of its noise level. Therefore, if a sample ion is absorbed to a low-conductivity water such as deionized water and 1 xcexcS water is prepared for example, because the above-described change becomes about xc2xd, detection may be possible. However, because such a measuring method is carried out at a repeated batch sampling formation, equipment and reagent therefor are required, and the measuring apparatus becomes expensive as well as the measuring operation becomes troublesome. Moreover, it is difficult to continuously measure the change in concentration.
Accordingly, an object of the present invention is to provide an ion concentration meter which can measure a change in ion concentration, such as of ammonia, with extremely high accuracy and sensitivity, which also can carry out continuous measurement, and which has a simple structure and can be manufactured at a low cost.
To accomplish the above object, an ion concentration meter according to the present invention comprises a difference conductivity meter wherein two conductivity measuring cells each having at least two electrodes are arranged in series in a flow path of a sample to be measured so that the sample being sent may make contact with the cells in sequence, the difference conductivity meter produces a difference between signals themselves detected by the conductivity measuring cells as a difference in conductivity of the sample between the positions of the conductivity measuring cells, and the ion concentration meter derives a change in ion concentration of the sample from the output from the difference conductivity meter, based on a predetermined correlation between a change in conductivity of the sample and a change in concentration of an ion to be detected in the sample.
In the present invention, although the ion to be detected is not particularly restricted, as ions capable of being effectively detected, at least one selected from the group consisting of ammonium, sodium, chloride, calcium, potassium, carbonate, silica, magnesium and sulphate ions can be cited.
In this ion concentration meter, it is preferred that a time delay column having a predetermined capacity is interposed between the above-described two conductivity measuring cells arranged in the flow path of the sample to be measured. Namely, at a condition where a time difference set by the time delay column is given, a difference between signals themselves detected by both conductivity measuring cells is produced, and based on the output, the change in ion concentration is measured. Although directly it is detected as a change in conductivity of a sample, since the correlation between a change in conductivity of the sample and a change in concentration of an ion to be detected in the sample has been determined in advance, by reading or by using a simple calculating means (a calculation program) based on the correlation, a change in ion concentration of the sample is easily derived.
Further, in the ion concentration meter, it is preferred that a degasifier capable of degasifying and defoaming the sample being sent is disposed upstream of the two conductivity measuring cells arranged in the flow path of the sample to be measured. By this, any influence of micro bubbles and the like on the measurement can be removed.
Further, the ion concentration meter can further comprise means for sending a sample to be measured with respect to a change in ion concentration to the flow path of the sample to be measured, and means for injecting a standard raw liquid into the sample to be measured with respect to a change in ion concentration. In such a structure, because it is possible to always compare the ion concentration of the sample with the standard raw liquid, for example, even if a change in ion concentration of the sample to be measured exhibits to be extremely minute during a short period of time and it is difficult to detect the minute change, in a case where the change is continued, when the change in ion concentration becomes more than a certain level after a certain time, the change can be surely detected. Further, a structure also can be employed for the ion concentration meter wherein a standard raw liquid, for example, a standard raw liquid having a constant ion concentration or substantially containing no ion is used as a carrier fluid, means for sending the carrier fluid to the flow path of the sample to be measured is provided, and while the sample to be measured with respect to a change in ion concentration is injected into the carrier fluid, the change in ion concentration of the sample is measured. In such a structure, because it is possible to always compare the ion concentration of the sample with the standard raw liquid, for example, even if a change in ion concentration of the sample to be measured exhibits to be extremely minute during a short period of time and it is difficult to detect the minute change, in a case where the change is continued, when the change in ion concentration becomes more than a certain level after a certain time, the change can be surely detected.
Further, the ion concentration meter also can be structured so as to further comprise means for switching a plurality of sample sources and sending a sample from a selected sample source to the flow path of the sample to be measured. In this structure, the path of the sample to be measured having the above-described two conductivity measuring cells can be disposed for each of the plurality of sample sources. Whichever structure is to be employed may be decided depending on the frequency or interval of measurement, the necessity of continuous measurement, etc.
Although the structure of the conductivity measuring cell itself is not particularly restricted, the following structures can be employed. For example, a structure can be employed wherein the above-described at least two electrodes in each conductivity measuring cell comprise a conductivity detection electrode and an electric current supply electrode. Alternatively, another structure can be employed wherein each of the conductivity measuring cells has three electrodes, the three electrodes include a conductivity detection electrode and two AC current supply electrodes disposed on both sides of the conductivity detection electrode at respective distances, and an AC current of the same phase is applied to the two AC current supply electrodes. Alternatively, a further structure can be employed wherein each of the conductivity measuring cells has three electrodes, the three electrodes include a conductivity detection electrode, an AC current supply electrode disposed on one side of the conductivity detection electrode at a distance, and a grounded electrode disposed on the other side of the conductivity detection electrode at a distance.
In such conductivity measuring cells, it is preferred that the above-described at least two electrodes are constructed so that their electrode surfaces are formed by titanium oxide layers on electrode bodies made of a conductive metal. In such a constitution, when organic substances and the like are contained in a sample to be measured, the property for decomposing organic substances based on the photocatalytic activity of the titanium oxide, and its super-hydrophilicity can be effectively utilized, in order to eliminate adverse effects on the measurement of the conductivity due to the adhesion or adsorption of the organic substances to the electrode surfaces. It is preferred that light irradiating means is disposed against the titanium oxide layers to provide a photocatalytic activity to the titanium oxide layers. For example, each conductivity measuring cell can be constructed so as to have a space for storing a substance to be measured defined between respective electrode surfaces of the above-described at least two electrodes, and light irradiating means that irradiates light onto the respective electrode surfaces.
In the conductivity measuring cells, it is preferred that light irradiated by the above-described light irradiating means has a wavelength which brings about a photocatalytic activity of the above-described titanium oxide layers. For example, light with a wavelength from about 300 to about 400 nm can be employed. As the light irradiating means, a light source composed of means for irradiating ultraviolet rays and the like such as a black light may be directly employed, and a light guiding material (for example, an optical fiber) to guide light from a light source provided as means for irradiating light may also be employed.
Further, the above-described space for storing a substance to be measured may be defined by a light transmitting material, and it may be constituted so that the light from the light irradiating means is irradiated onto an electrode surface through the light transmitting material (for example, glass). In this case, if a titanium oxide coating layer capable of transmitting light is provided on the surface of the light transmitting material at its side facing the space for storing a substance to be measured (a surface in contact with solution), adhesion of organic substances and the like to this surface of the light transmitting material can be prevented by super-hydrophilicity and organics decomposition property ascribed to the titanium oxide layer.
The above-described electrode can be produced by, for example, the following method. Namely, a method can be employed wherein an electrode surface is formed by providing on a titanium oxide layer on a surface of an electrode body made of a conductive metal by a surface treatment such as sputtering, plating or the like. Alternatively, a method can also be employed wherein an electrode surface made of a titanium oxide layer is formed by providing oxygen to a surface of an electrode body made of titanium. As the method for forming a titanium oxide layer by providing oxygen, a method based on air oxidation other than a method utilizing electrolysis can be employed.
Such an ion concentration meter according to the present invention is suitable for application to the measurement of a change in ion concentration of a fluid to be heat exchanged in a heat exchange system, or for application to the measurement of a change in ion concentration of a liquid diluted or mixed. Further, the ion concentration meter according to the present invention is suitable for application to a cooling water producing system. For example, the ion concentration meter can be constituted as a meter wherein the sample is collected from a brine in a cooling water producing system, and the ion concentration meter measures a change in concentration of ammonia which has leaked from the side of a refrigerator into the brine.
In the ion concentration meter according to the present invention, basically, not an absolute value of a conductivity of a sample but a change in conductivity is measured, and the measured change in conductivity is determined as a value corresponding to a change in ion concentration. Since a value of a change is detected, an extremely high-accuracy measurement becomes possible in spite of a large base value of the conductivity or the ion concentration. Further, since a sample can be directly measured, equipment and reagent for the measurement at a repeated batch sampling formation as in the conventional method are not required, and therefore, the measuring apparatus and the operation can be both simplified. Furthermore, if the electrodes utilizing the photocatalytic activity of titanium oxide are used, a stable measurement can be possible.